This invention relates to apparatus for terminating the last stitch at a predetermined point using a workpiece-end detector and, more particularly, this invention is applicable where a sewing machine is running at high speed of around 5,000 s.p.m. (strokes per minute).
According to the conventional type of apparatus for terminating the last stitch, a workpiece-end detector is located in the counter-feed direction from the needle entry, and when the workpiece-end detector detects the end of workpiece, the detector sends signals to reduce the stitching speed and to adjust the current stitch pitch so that the last stitch terminates exactly at a predetermined point.
Referring to FIG. 2B, when the workpiece-end detector 19 detects the end of the workpiece, considering to the distance M which is between the needle entry 11 and the workpiece-end detector 19, margin "a" of predetermined point E, and the current stitch pitch P, the required number of stitches to terminate the last stitch at the predetermined point E will be operated, namely (M-a)/P. This operated number of stitches, (M-a)/P, should be larger than the number of stitches required to stop the machine by reducing the speed gradually. It is impossible to terminate the last stitch at the predetermined point within less than the number of stitches required with speed gradually decreasing from the view point of mechanical construction, so the (M-a)/P value must be larger than this required number of stitches.
According to test results, when the sewing machine is running at 1500 s.p.m., at lease five more stitches are required to stop the machine. In such a case, the required number of stitches for stopping is five. However, the larger the (M-a)/P value, the larger the summation of errors per each stitch pitch will become. Thus, the last stitch will deviate from the predetermined point.
Such difficulties illustrate a contradiction in the prior art relating to termination of the last stitch at a predetermined point. Although the stitch pitch was adjusted from 2mm to 1.5mm, the adjusted stitch pitch may not stitch at a 1.5mm pitch because the workpiece stretches due to its elastic nature. Therefore, the actual stitch pitch may be 1.3mm. In such a case, if (M-a)/P, namely the number of stitches, is large, the last stitch will deviate far from the predetermined point. High speed performance of the sewing machine is of course desirable. However, the more the stitching speed increases, the more the (M-a)/P value increases. On the other hand, in order to terminate the last stitch at the predetermined point precisely, the value (M-a)/P should be small. Thus, there is a contradiction.
Considering the above points, although the sewing machine is capable of operating at 5,000 s.p.m. (strokes per minute), the sewing machine must be operated at 3,000 s.p.m. to effectively operate the apparatus for terminating the last stitch at a predetermined point. As a result, where the stitch line is long, time is wasted.